Photodetectors

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Chapter 6

• Photodetectors

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Content

• Physical Principles of Photodiodes
• pin, APD
• Photodetectors characteristics (Quantum
  efficiency, Responsivity, S/N)
• Noise in Photodetector Circuits
• Photodiode Response Time
• Photodiodes structures

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pin Photodetector
w
The high electric field present in the depletion
region causes photo-generated carriers to
Separate and be collected across the reverse
biased junction. This give rise to a current
Flow in an external circuit, known as
photocurrent.
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Energy-Band diagram for a pin photodiode
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Photocurrent

• Optical power absorbed, in the depletion
  region can be written in terms of incident
  optical power,
• Absorption coefficient strongly depends
  on wavelength. The upper wavelength cutoff for
  any semiconductor can be determined by its energy
  gap as follows
• Taking entrance face reflectivity into
  consideration, the absorbed power in the width of
  depletion region, w, becomes

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Optical Absorption Coefficient
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Responsivity

• The primary photocurrent resulting from
  absorption is
• Quantum Efficiency
• Responsivity

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Responsivity vs. wavelength
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Avalanche Photodiode (APD)
APDs internally multiply the primary photocurrent
before it enters to following circuitry. In
order to carrier multiplication take place, the
photogenerated carriers must traverse along a
high field region. In this region, photogenerated
electrons and holes gain enough energy to ionize
bound electrons in VB upon colliding with them.
This multiplication is known as impact
ionization. The newly created carriers in the
presence of high electric field result in more
ionization called avalanche effect.
Optical radiation
Reach-Through APD structure (RAPD) showing the
electric fields in depletion region and
multiplication region.
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Responsivity of APD

• The multiplication factor (current gain) M for
  all carriers generated in the photodiode is
  defined as
• Where is the average value of the total
  multiplied output current is the primary
  photocurrent.
• The responsivity of APD can be calculated by
  considering the current gain as

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Current gain (M) vs. Voltage for different
optical wavelengths
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Photodetector Noise S/N

• Detection of weak optical signal requires that
  the photodetector and its following amplification
  circuitry be optimized for a desired
  signal-to-noise ratio.
• It is the noise current which determines the
  minimum optical power level that can be detected.
  This minimum detectable optical power defines the
  sensitivity of photodetector. That is the optical
  power that generates a photocurrent with the
  amplitude equal to that of the total noise
  current (S/N1)

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Signal Calculation

• Consider the modulated optical power signal P(t)
  falls on the photodetector with the form of
• Where s(t) is message electrical signal and m is
  modulation index. Therefore the primary
  photocurrent is (for pin photodiode M1)
• The root mean square signal current is then

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Noise Sources in Photodetecors

• The principal noises associated with
  photodetectors are
• 1- Quantum (Shot) noise arises from
  statistical nature of the production and
  collection of photo-generated electrons upon
  optical illumination. It has been shown that the
  statistics follow a Poisson process.
• 2- Dark current noise is the current that
  continues to flow through the bias circuit in the
  absence of the light. This is the combination of
  bulk dark current, which is due to thermally
  generated e and h in the pn junction, and the
  surface dark current, due to surface defects,
  bias voltage and surface area.
• In order to calculate the total noise presented
  in photodetector, we should sum up the root mean
  square of each noise current by assuming that
  those are uncorrelated.
• Total photodetector noise currentquantum noise
  current bulk dark current noise surface
  current noise

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Noise calculation (1)

• Quantum noise current (lower limit on the
  sensitivity)
• B Bandwidth, F(M) is the noise figure and
  generally is
• Bulk dark current noise
• is bulk dark current
• Surface dark current noise is the
  surface current.

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Note that for pin photodiode
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Noise calculation (2)

• The total rms photodetector noise current is
• The thermal noise of amplifier connected to the
  photodetector is
• input resistance of amplifier, and
  is Boltzmann cte.

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S/N Calculation

• Having obtained the signal and total noise, the
  signal-to-noise-ratio can be written as
• Since the noise figure F(M) increases with M,
  there always exists an optimum value of M that
  maximizes the S/N. For sinusoidally modulated
  signal with m1 and

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Photodetector Response Time

• The response time of a photodetector with its
  output circuit depends mainly on the following
  three factors
• 1- The transit time of the photocarriers in
  the depletion region. The transit time
  depends on the carrier drift velocity and
  the depletion layer width w, and is given by
• 2- Diffusion time of photocarriers outside
  depletion region.
• 3- RC time constant of the circuit. The
  circuit after the photodetector acts like RC low
  pass filter with a passband given by
  

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Photodiode response to optical pulse
Typical response time of the photodiode that is
not fully depleted
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Various optical responses of photodetectors
Trade-off between quantum efficiency response
time

• To achieve a high quantum efficiency, the
  depletion layer width must be larger than
• (the inverse of the absorption
  coefficient), so that most of the light will be
  absorbed. At the same time with large width, the
  capacitance is small and RC time constant getting
  smaller, leading to faster response, but wide
  width results in larger transit time in the
  depletion region. Therefore there is a trade-off
  between width and QE. It is shown that the best
  is

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Structures for InGaAs APDs

• Separate-absorption-and multiplication (SAM) APD
• InGaAs APD superlattice structure (The
  multiplication region is composed of several
  layers of InAlGaAs quantum wells separated by
  InAlAs barrier layers.

light
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Temperature effect on avalanche gain
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Comparison of photodetectors